The phenomena related with High Voltage Direct Current (HVDC) converter connected to weak AC network have been extensively studied. See for instance “Guide for planning DC links terminating at AC locations having Low Short-Circuit capacities”—Part I: AC/DC interaction phenomena’, CIGRE working group 14.07, Report 68, June 1992 and “IEEE guide for planning DC links terminating at AC locations having Low Short-Circuit capacities”, IEEE Std 1204-1997
These phenomena have different implications on the design of HVDC system. The following issues are of special concern:                Risk for voltage/power instability        High temporary over-voltages        Low frequency resonances        Long restart times        Risk for commutation failures        
All these factors influence the system performance, and to mitigate their effects it might be necessary to install additional equipment like static or synchronous compensators, which increases the overall cost of the installed equipment.
A measure of the strength of the AC system is given by the short-circuit ratio, SCR. This is defined as the ratio between the short-circuit capacity of the AC network at the commutation bus and the nominal DC power level. An AC system is usually regarded as weak if SCR is lower than 3.
The Voltage/Power stability is a basic issue of concern when HVDC converters with line commutated converters are connected to a weak AC system. The basic mechanism in unstable situations is the inability of the connected power system to provide the reactive power needed by the converter to maintain acceptable system voltage.
Traditional design of a line commutated inverter assumes operation with constant extinction angle γ. This design allows the operation of the converters with minimum reactive power consumption, and reduced amount of reactive power shunt compensation. Alternatively, other operating properties can also be found like constant DC voltage or constant firing angle α. However, all these operating conditions imply similar limiting conditions in terms of strength of the connected AC system.
To mitigate the interaction between converter and the connected AC system a converter that uses Capacitor Commutated Converter (CCC converter) has been developed. The CCC converter is a classical thyristor based converter provided with series capacitor placed between the converter valves and converter transformer. The series capacitors used in these converters are often denoted commutation capacitors. This type of converter is for instance described by T. Jonsson and P. E. Björklund in “Capacitor Commutated Converters for HVDC”, IEEE PES PowerTech conference, Stockholm, pp. 44-51, June 1995.
According to the article by Jonsson and Björlund the CCC converter has improved performance compared to a conventional HVDC converter, especially when the CCC converter is connected to a weak AC system. Studies have demonstrated that it may be possible to connect these types of converters to AC systems that are as weak as having SCR=1. It should here be noted that conventional HVDC systems with line commutated converter and converters without assistances from series capacitors are limited to SCR≈2.
However there is still a need for improvement in operation of both types of converters, conventional and CCC, when the AC system to which they are connected is weak.